1. Field of the Invention
This invention relates in general to an apparatus for manufacturing a semiconductor material, and more particularly to an apparatus for manufacturing a semiconductor material which can keep a pressure difference between in a process furnace and in a transfer chamber to be small.
2. Related Art
A single wafer apparatus for manufacturing a semiconductor material wherein wafers are processed one by one, such as for a thermal process or a vapor phase growth of a layer on a wafer, includes a process furnace for conducting the above processes and so on to a semiconductor substrate or a wafer, a load-lock chamber for taking the wafer into or out of the apparatus, and a transfer chamber for transferring the wafer from the load-lock chamber to the process furnace or from the process furnace to the load-lock chamber.
FIG. 2 shows a conventional single wafer apparatus for manufacturing a semiconductor material. In this apparatus, a process furnace 1 is connected to a transfer chamber 2 through a gate valve 4, and the transfer chamber 2 is connected to a load-lock chamber 3 through a gate valve 5.
The load-lock chamber 3, which is provided for taking a wafer 6 into or out of the apparatus, can contain one cassette which can hold up to 25 wafers of 8 inches for example. The atmosphere of the load-lock chamber 3 is usually kept in nitrogen. The nitrogen gas is supplied into the load-lock chamber 3 from an introducing pipe 21 at a flow rate of 15 liters/min.
The transfer chamber 2 is a chamber for transferring the wafer 6 from the load-lock chamber 3 to the process furnace 1, and provided with a transfer instrument for transferring the wafer 6, though not shown in FIG. 2. The transfer instrument takes the wafer 6 one by one from the cassette 10 in the load-lock chamber 3 and transfers the wafer 6 into the transfer chamber 2 where the wafer 6 is kept still until the process for the other wafer in the process furnace 1 is completed. When the process for the other wafer is completed in the process furnace 1, the gate valve 4 is opened and the wafer 6 in the transfer chamber 2 is transferred to the process furnace 1. The atmosphere in the transfer chamber 2 is also kept in nitrogen. The flow rate of nitrogen gas supplied from an introducing pipe 22 to the transfer chamber 2 is kept at 15 liters /min. by a restrictor 7. The nitrogen gas is then discharged to the air through an exhaustion pipe 14 provided with a butterfly valve 13.
The process furnace 1 is a furnace for conducting an aimed process to the wafer 6. If the apparatus is a vapor phase growth apparatus, for example, a vapor phase epitaxial growth is conducted in the process furnace 1. In this case, the atmosphere in the process furnace 1 is always kept in hydrogen by supplying hydrogen from an introducing pipe 23 at a flow rate of 40-60 liters/min., and etching gas, source gas, dopant gas and so on are occasionally supplied into the process furnace 1 with hydrogen at 700-1200.degree. C. The gases supplied into the process furnace 1 are flown through an exhaustion pipe 19 to a gas scrubber 18, where the etching gas, the source gas, the dopant gas and so on are completely removed from the exhaustion gas and then the hydrogen gas is discharged into the air.
In the above described apparatus, in order to avoid escaping of the process gas in the process furnace 1 to the transfer chamber 2 and the load-lock chamber 3, relations among the pressures P's in the chambers and the furnace are preferably: EQU P(process furnace 1)&lt;P(transfer chamber 2)&lt;P(load-lock chamber 3)
Further, in order to avoid generation of rapid flows from one chamber or furnace to the other due to the pressure differences therebetween, bypass lines 11 and 15 and auto-valves 12 and 16 are provided so that the gate valves are set to be opened after the pressures in the chambers are even by opening the auto-valves just before the gate valves open. Additionally, manometers 25 and 26 are provided for measuring the pressures in the transfer chamber 2 and the process furnace 1, which are kept at 740-760 torr respectively.
According to the apparatus, the wafer 6 is processed in a following way. First, a door of the load-lock chamber 3 is opened, a cassette 10 holding the wafers 6 is set in the load-lock chamber 3, and then the atmosphere in the load-lock chamber 3 is substituted to nitrogen by vacuuming. After that, no pressure difference between the load-lock chamber 3 and the transfer chamber 2 are made by the bypass line 11 beforehand, and then the gate valve 5 of the load-lock chamber 3 is opened, and one of the wafers 6 is transferred to the transfer chamber 2 by the transfer instrument in the transfer chamber 2, and then the gate valve 5 is closed. Then, after the pressure difference between in the transfer chamber 2 and the process furnace 1 is made small by the bypass line 15 beforehand, and then the gate valve 4 is opened, and the wafer 6 is transferred to the process furnace 1. Then, the gate valve 4 is closed. After a predetermined process for the wafer 6 is conducted in the process furnace 1, the wafer 6 goes back to the load-lock chamber 3 from the process furnace 1 by returning the reverse pass with a similar sequence.
However, according to the conventional apparatus, there are problems described below. When the wafer 6 is transferred from the transfer chamber 2 to the process furnace 1, no pressure difference between in the process furnace 1 and in the transfer chamber 2 is made by opening the auto-valve 16 of the bypass line 15 before opening the gate valve 4, however, if the pressure difference is large, there generates a rapid gas flow in the bypass line 15 when the auto-valve 16 of the bypass line 15 opens due to the large pressure difference.
The rapid gas flow flows into the process furnace 1 if the pressure in the transfer chamber 2 is higher than that in the process furnace 1, and flows into the transfer chamber 2 if the pressure in the process furnace 1 is higher. Consequently, particles existing in the process furnace 1 or in the transfer chamber 2 are blown up thereby. The gate valve 4 is opened few seconds after the auto-valve 16 is opened, however, the blowing up of the particles continues for dozens of seconds, consequently there may occur contamination of the wafer 6 by the particles. The wafer 6 may be contaminated by the particles of about 80 particles/wafer if the pressure difference between in the process furnace 1 and in the transfer chamber 2 is 7 torr or more, for example.
Further, in the process furnace 1, the gas flow from the bypass line 15 does not flow directly forward the front surface side of the wafer 6 since the end of the bypass line 15 is provided below the back side of a susceptor 17 in the process furnace 1, however, the gas flow may reach to the front surface side of the wafer 6 from the back side surface of the susceptor 17 if the gas flow is rapid, and the gas flow may blow up polysilicon deposited on the front surface side of the wafer 6 as particles.
Further, a pipe of the bypass line 15 is relatively narrow, and it takes some time to make the pressures in the chamber or the furnace same, even if the auto-valve 16 of the bypass line 15 is opened, therefore the pressures in the chamber and the furnace do not become the same before the gate valve 4 is opened, if the pressures are widely different between in the process furnace 1 and the transfer chamber 2. Consequently, there may generate a rapid gas flow which flows through the gate valve 4 just when it is opened, and it may cause blowing up of particles.
The pressure difference between in the process furnace 1 and the transfer chamber 2 is caused by mainly following 2 reasons.
The first reason is an existence of the pressure difference between in the exhaustion pipes 19 and 14 respectively provided to the process furnace 1 and the transfer chamber 2. The exhaustion pipe 14 provided to the transfer chamber 2 is open directly to the air through the butterfly valve 13. On the other hand, the exhaustion pipe 19 provided to the process furnace 1 is open to the air through the gas scrubber 18. Therefore, the pressure in the exhaustion pipe 19 of the process furnace 1 is generally lower than that in the exhaustion pipe 13 of the transfer chamber 2 by 7 torr or more.
The second reason is a choking of the exhaustion pipe 19 provided to the process furnace 1. Products generated by a reaction between the non reacted source gas and the other gases are deposited in the exhaustion pipe 19, which may choke the pipe. Consequently, the pressure in the process furnace 1 connected with the exhaustion pipe 19 gradually increases, and finally becomes higher than the pressure in the transfer chamber 2 connected with the exhaustion pipe 14 by 2 torr or more.
Thus, contamination of the wafer 6 occurs due to the blowing up of particles when the pressure difference between in the process furnace 1 and the transfer chamber 2 becomes large. Further, if the pressure in the process furnace 1 is higher than that in the transfer chamber 2, the etching gas or residual source gas in the process furnace 1 flows into the transfer chamber 2. Such corrosive gases may corrode the transfer instrument in the transfer chamber 2.